The present invention relates to a polymer alloy including polylactic acid, and particularly to a novel commodity resin composition which hardly adversely affects the environment. The resin composition of the present invention is used for preparing various molded articles as commodity resins are used therefor.
Polylactic acid is a colorless, transparent polyester prepared using, as a starting material, lactic acid which is a biomass, and is known as a plastic material which hardly causes adverse effects on environments in disposal (for example, incineration) of waste products such as molded articles. Therefore, use of it as commodity resins for preparing various molded articles is now under consideration.
Polylactic resins have problems in physical properties, for example, (1) they are low in shock-proof because they are hard and fragile, (2) they are apt to undergo thermal transformation because the glass transition temperature is not so high (about 60° C.), and (3) they are apt to cause hydraulic degradation in environment of high-temperature and high-humidity. It is considered that these problems hinder the application and practical utilization of them as materials which are required to have a long lifetime.
The above problem (1) on shock-proof can be solved by mixing the polylactic resins with aliphatic polyesters which have low melting point and are soft, such as polybutylene succinate (PBS) (cf. JP-A-8-245866). The above problem (2) on thermal transformation can be solved by dispersing a nucleator such as fine montmorillonite to make the polylactic acid easy in crystallization (cf. Suprakas Sinha Ray, et al., “Macromolecules”, 35, p3104, 2002). However, as to the problem (3), effective solution has not yet been found.
On the other hand, Ajay M. Gajria, et al have proved that a mixture of polylactic acid and polyvinyl acetate at 7:3 has a decomposition rate which is 1/200 or less that of pure polylactic acid in hydrolysis test using an enzyme in an environment of 37° C. (cf. “Polymer”, 37, p437, 1996). The authors have considered that the reduction of decomposition rate is due to the decrease of deposition of decomposition enzyme on molded articles caused by mixing polylactic acid with polyvinyl acetate which lowers surface tension.
However, this technology cannot completely prevent the hydrolysis, and the hydrolysis proceeds even in the absence of enzyme in an environment of further higher temperature and humidity (water content). Thus, this measure is insufficient for solution of the problem. Moreover, commodity plastics such as polystyrene and polypropylene which are expected to provide a long lifetime even in the environment of high temperature and humidity have no oxygen atom in the molecule, and hence they have no compatibility with polyesters having oxygen atom in the molecule such as polylactic acid and cannot be admixed with polylactic acid. Therefore, further countermeasures are necessary to use polylactic acid in the same manner as commodity plastics.
The object of the present invention is to obtain a resin composition excellent in shock-proof and thermal transformation resistance and having sufficiently lowered hydrolyzability by blending polylactic acid with a suitable resin component, and thus the present invention provides a polymer alloy including polylactic acid which can be used in the same manner as conventional commodity plastics even in environment of high temperature and high humidity.